Cardinal mark

A cardinal mark is a sea mark (a buoy or other floating or fixed structure) commonly used in maritime pilotage to indicate the position of a hazard and the direction of safe water.

This makes them meaningful regardless of the direction or position of the approaching vessel, in contrast to the (perhaps better-known) lateral mark system.

The characteristics and meanings of cardinal marks are as defined by the International Association of Lighthouse Authorities.

A cardinal mark indicates one of the four compass directions by: Either a quick or a very quick sequence of light flashes may be used; the choice allows for two similar nearby marks to be uniquely identified by their lights.

The colours can be remembered this way: The two conical top-marks always point to black.